1. Technical Field
The present invention relates to a device for sensing malfunctions in a refrigerant compressor and disengaging the compressor from its driving source upon detection of a malfunction. More particularly, the present invention includes a device for overriding the malfunction sensor during engine start-up.
2. Background Information
Automotive air conditioning systems typically include a compressor engageable to the driving force of the automobile's engine via an electromagnetic clutch. As is well known in the art, the shaft of the compressor extends through the center of a pulley of the clutch and is affixed to an armature plate of the clutch. A V-shaped belt is connected between a pulley connected to the shaft of the engine and the pulley of the electromagnetic clutch. In order to cause the shaft of the compressor to rotate, current is supplied to a coil which is wrapped about the clutch between the pulley and the compressor. This current sets up a field which attracts and frictionally engages the armature plate to a portion of the driven pulley, thereby rotating the shaft of the compressor.
The compressor can malfunction, for example due to lubrication problems or component breakage, causing the compressor shaft to stop rotating. Upon such malfunction, slippage between the belt and the clutch pulley is produced, causing rapid wear and early breakage of the belt. In applications where the clutch belt also drives other automotive components, such as a generator, oil pump for power steering, or a water pump, sudden and unexpected belt breakage can have serious repercussions. In order to eliminate the rapid wear of the belt due to compressor malfunction, prior art devices are known, such as that shown in FIG. 1, which sense the compressor rotation and disengage the electromagnetic clutch upon sensing compressor malfunction.
As shown in FIG. 1, the driving force of engine 1 is transmitted to electromagnetic clutch 3 via belt 2, causing compressor 4 to rotate when clutch 3 is engaged, as explained above. The rotation of compressor 4 is detected by revolution sensor 5, which is typically built into the housing of the compressor and senses a rotating magnetic portion of the compressor. Sensor 5 usually includes an electromagnetic coil wound about a permanent magnet; rotation of the magnetic compressor portion causes the density of the magnetic flux formed between the permanent magnet and the rotating portion to change. Consequently, pulses, shown in FIG. 2, are generated from the coil at a period indicative of the rotational speed of compressor 4. The pulses from sensor 5 are rectified by rectifier 6, and the rectified pulses are counted by counter 7, which counts the number of pulses produced within a predetermined time period. If the number of pulses is less than a predetermined number, the compressor is rotating at a slower-than-expected rate, and a malfunction is assumed. Upon such malfunction, counter 7 generates a malfunction signal to control circuit 8, which disengages the compressor from the clutch by deenergizing the clutch coil via relay (not shown). The de-energized coil releases the armature plate, taking the compressor out of the system, thereby preventing damage to those automotive components driven by belt 2.
One of the disadvantages of the malfunction detector of FIG. 1 occurs when the engine is off but the switch of the air conditioning apparatus is in the "ON" position. When the ignition switch is turned on to start the engine, electric current is applied to the coil of the electromagnetic clutch, engaging the compressor. However, the starting motor cannot easily start the engine with the compressor engaged. Accordingly, the engine turns at a slower rate, and the compressor generates pulses at less than the predetermined rate, causing the control circuit to erroneously disengage the compressor.
One solution to the above problem is sensing both engine and compressor rotation, as shown for example in U.S. Pat. No. 4,462,491 issued to Kono et al. and herein incorporated by reference. However, these types of detection devices are expensive, due in part to the requirement of two sensors, and complex circuitry is also required to co-ordinate the two sensors.